This invention relates to heat exchangers and more particularly, to heat exchangers of the type in which tube sections are disposed within a shell, duct or chamber, such as vapor generators, feedwater heaters, condensers, air preheaters, etc., wherein standing wave vibration can occur.
There are several causes for the creation of standing wave vibration, including a phenomenon known as "vortex shedding." When a fluid flows over the outside surface of tubes, the fluid will form into vortices, spinning in alternating clockwise and counterclockwise directions on opposite sides of the tubes after passing over the tube surfaces. The vortices form in a regular and periodic manner. When the tubes are disposed within a shell or chamber, the vortices create pressure pulsations which affect "columns" of the fluid which extend transversely of the direction of fluid flow in the direction perpendicular to the tube axes. The particles comprising the fluid columns are excited by the pressure pulsations, and can form into standing waves of particular frequencies. The frequency of a standing wave is a function of the width of the fluid column, being inversely proportional thereto and calculated in accordance with the following formula: ##EQU1## where f=frequency in Hz,
n=1, 2, 3, . . ., or other integer, representing the mode number, PA1 C=dimensional constant, PA1 T=fluid temperature in degrees Rankine, and PA1 W=width of the fluid column in feet.
When the excitation frequency (the frequency of the pressure pulsations) approaches or equals the natural frequency of a particular fluid column (the standing wave frequency for that column), that is, when resonance occurs, the fluid column will vibrate and depending upon the amplitude or intensity and frequency of the standing wave can manifest itself as noise.
It should be understood that vortex shedding is only one cause for standing wave vibration, and that several other causes exist, including fluid flow turbulence and tube vibration. Vortex shedding, fluid flow turbulence, and/or tube vibration can each result in pressure pulsations which propagate through the fluid and affect fluid columns in such a manner as to cause standing wave vibration. When two or more such causes occur simultaneously, for example if tube vibration occurred simultaneously with vortex shedding, a "coupling" effect can occur whereby tube vibration and vortex shedding will together contribute to the creation of standing wave vibration.
In addition to the creation of noise, standing wave vibration may cause vibration of the shell or chamber boundary walls, and can even result in damage to the tube elements.
In the past, standing wave vibration was prevented by installing baffles within the shell, duct or chamber which extended parallel to the direction of fluid flowing over the tube outer surfaces and perpendicular to the direction of the standing waves. This technique had the effect of altering the standing wave frequency by changing the dimension W, previously discussed. Baffles took the form of metal sheets disposed between longitudinally extending rows of tubes, and were fastened at their edges to the walls of the shell or even to the tubes themselves. Unfortunately, baffles may buckle, burn, or corrode when exposed to fluids at high temperatures. Furthermore, in some cases the orientation of the tubes forming a tube bank may not allow for locating baffles between tubes, which would be true for example in the case of the staggered tube array shown in FIG. 3 hereinafter. Additionally, the number of baffles that may be required to prevent standing wave vibration may be so high as to make the use of baffles quite costly.
The instant invention provides a method of operating a heat exchanger by which standing wave vibration is minimized, which can be employed regardless of the orientation of the tubes, and which prevents against the problems of buckling, burning, or corroding associated with baffles. Additionally, when compared against heat exchangers incorporating baffles, the invention is relatively inexpensive.